The invention relates to high voltage semiconductor packages, and particularly to ones especially suited for containing high voltage isolation amplifiers, and more particularly to such packages utilizing fringe capacitors as small signal isolation barriers between input and output portions of isolation amplifiers.
There are many applications for a type of amplifier circuit referred to by those skilled in the art as an "isolation amplifier". Isolation amplifier circuits have electrically isolated input and output stages that are separated by "isolation barriers" that can withstand voltage differences of at least hundreds of volts, and in some cases thousands of volts, and are capable of amplifying small AC input signals to produce larger AC output signals despite the large DC or common mode voltage difference between the input and output terminals. Typical applications for isolation amplifiers include industrial measurement systems, medical electronic equipment, electronic test equipment, and numerous other applications where highly isolated signal transmission is needed. Isolation amplifiers are generally considered to be expensive components. The extent of utilization of isolation amplifiers in the electronics industry would be much greater if lower cost devices of this type could be manufactured. Up to now, however, no one has been able to make a highly accurate, high voltage isolation amplifier, especially one that is hermetically sealed, inexpensively enough to "open up the market" for widespread use of such devices in low cost electronics products.
In the past, most isolation amplifiers either have utilized toroidal transformers as isolation barriers, or else have used optoelectronic devices as isolation barriers. Optoelectronic devices at the present state of the art are too expensive or too slow for many applications, although they offer a high degree of electrical isolation between the input and output stages thereof. Isolation amplifiers utilizing ferrite toroid transformers as isolation barriers are large in size, and are difficult to integrate into hybiid integrated circuit packages, and are very expensive. No hermetically sealed hybrid integrated circuit isolation amplifiers of this type are yet commercially available.
One patent, commonly assigned U.S. Pat. No. 4,292,595 (Smith) introduces the concept of using capacitors as isolation barriers for high voltage isolation amplifiers. The technique described requires use of large (50 picofarad) capacitors which would occupy a large amount of area on a hybrid integrated circuit substrate, and is impractical.
Prior isolation amplifiers utilizing separate toroidal transformers for coupling small AC signals across an isolation barrier and simultaneously coupling large high power DC signals across an isolation barrier between the same input and output stages are known. Such circuits are expensive.
Fringe capacitors have been described in the prior art. For example, U.S. Pat. Nos. 4,188,651 (Dornfeld, et al.), 3,764,938 (Barnes), 3,675,095 (Lehmann), and 3,104,377 (Alexander et al.) disclose interdigitated coplanar capacitor structures. None of these, however, is disclosed as a high voltage component, nor would any of them be useful in a high voltage (i.e., greater than 1500 volts) isolation barrier structure for an isolation amplifier). Furthermore, none would be compatible with conventional hybrid integrated circuit manufacturing processes.
A variety of multiple cavity integrated circuit packages are known in the art, for example as shown in U.S. Pat. No. 4,038,488 (Lin). The structure disclosed in this reference is not useful for isolation amplifiers, because there is no isolation barrier between the two cavities. The stated purpose of the structure is to avoid any electrical coupling between the two cavities.